Previously considered uncommon, cerebrovascular accidents, and in particular brain infarcts, are increasingly recognized in
dogs or cats with the advance of neuro-imaging. The improved resolution of imaging methods has made the identification of
small pathologic process such as lacunar infarcts possible. These infarcts are now clearly the most common category of infarcts
in dogs. Initial hopes that MRI signal intensity would be diagnostic for specific pathologic processes have not been borne
out. Radiologists and neurologists have long tried to correlate histopathologic results and magnetic resonance signal intensity
using combination of sequences to distinguish non-neoplastic from neoplastic disease and to further classify different types
of neoplastic disease. The combined results of sequences might in some cases be strongly suggestive of a specific pathologic
process. However, pathologic processes, such as inflammatory mass and neoplasia, share some MRI characteristics as well as
similarities in origin, shape or anatomic site.
In many cases, the interpretation of the imaging findings rely on the clinical understanding of the suspected disease process
(differential diagnosis), the use of other diagnostic tests (cerebrospinal fluid analysis, antibody titers, metastatic work-up,
coagulation profile, etc.), a good understanding of the disease process mechanism, and/or histological diagnosis after tissue
biopsy (ultrasonographic guided, stereotactic or surgical). Magnetic resonance techniques may still hold the key to non-invasive
specific histological diagnosis. Magnetic resonance spectroscopy (MRS) provides metabolic information about brain tumors beyond
what is obtained from anatomic images. Response to radiation therapy is also reflected by MRS patterns. MRS is also of use
in the investigation of intrinsic metabolic disease where, while it is usually of insufficient sensitivity to measure the
specific chemical responsible for the disorder, it can detect the secondary chemical pathologic changes. In some situations,
these secondary chemical changes may be specific for the disorder.
The absence of abnormalities on MRI evaluation of the brain could indicate a. incorrect anatomical diagnosis (disease affecting
parts of the nervous system other than the brain), b. toxicity or metabolic disease affecting the brain function without causing
macroscopic parenchymal changes (hepatic encephalopathy, electrolyte imbalance or hypoglycemia), c. diseases causing very
subtle parenchymal changes that might not be detected with a low-field magnet (small infarct or hemorrhage and degenerative
disease), or d. functional brain disease (primary epilepsy and movement disorders).
Spinal cord diseases: common indication
Diseases affecting the spinal cord are broadly divided into compressive diseases (disk herniation, vertebral fracture/luxation,
spinal malformation, neoplasm) and non-compressive diseases (spinal cord malformation, inflammatory/infectious CNS disease,
degenerative diseases and vascular accident). Suspected spinal cord disease is a common indication for MRI in small animals.
Traditionally, survey radiography and myelography have been the techniques of choice for the investigation of spinal cord
disease. However, myelography provides no information about the spinal cord parenchyma other than whether it is compressed,
displaced or swollen. MRI offers the double advantage of showing spinal cord parenchymal changes (such as those associated
with inflammation, edema, syringomyelia-hydromyelia, infarct or neoplasia) and allowing transverse images (greatly assisting
with the localization of the compressive tissue). MRI also has the benefit of being safer than conventional myelography as
a subarachnoid injection is not required. Without an accurate anatomic diagnosis (suspected spinal cord segments affected),
MRI evaluation of the spinal cord can be extremely time consuming, especially in large dogs, and can lead to over-diagnosis
of incidental changes such as mildly herniated disks. Other limitations are mainly technical (magnetic field strength, field
of view allowed, accurate positioning of the animal, and setting of different imaging planes).
MRI evaluation of the spinal cord could be normal in the case of a. incorrect anatomical diagnosis (particularly with neuromuscular
disease, brain disease, or disease affecting other spinal cord segments than the ones imaged), b. diseases for which the parenchymal
changes can only been seen microscopically (especially degenerative disease such as degenerative myelopathy), or c. diseases
causing very subtle parenchymal changes that might not be detected with a low strength magnet (vascular disease such as fibrocartilaginous
embolism or inflammatory disease of the spinal cord).
Looking into nerve and muscle diseases
The peripheral nervous system consists of 12 pairs of cranial nerves and 36 pairs of spinal nerves that extend from, or to,
the spinal cord and brainstem. Diseases affecting the peripheral nervous system can affect one single peripheral nerve (mononeuropathy)
or multiple peripheral nerves (polyneuropathy). Common indications of MRI in investigation of peripheral nerve disease include
the investigation of a. cauda equina syndrome (very often technically difficult using myelography and epidurography), b. brachial
plexopathy, and c. cranial nerve neuropathy. The main limitations of MRI in these cases are mainly technical (choice of adequate
imaging plane and sequences), operator-dependent (detailed knowledge of anatomy), and inherent poor specificity in differentiating
neoplastic from non-neoplastic disease processes.